Radio-frequency circuit



March 18, 1952 F c, [SELY 2,589,259

' RADIO-FREQUENCY CIRCUIT Filed Aug. 15, 1948 INVENTOR. FRANK C ISELYATTORNEY Patented Mar. 18, 1952 UNITED STATES PATENT OFFICERADIO-FREQUENCY CIRCUIT Frank C. Isely, Washington, D. 0.

Application- August 13, 1948, Serial No. 44,210

(Granted under" the act of March 3, 1883, as amended April 30, 1928; 3700. G. 757) 5 Claims.

The present invention relates to an improvemen't'of the inventiondisclosed in my co-pending application Serial No. 353959, filed June 29,1948, now U'. S. Patent'2,520,147, and is in particular concerned withthe problem of eliminating unwanted harmonics in line controlled radiofrequen'cycircuits.

One serious disadvantage often experienced in the'use'of'line'controlled radio frequency circuits (distributed constantcircuits) is that such circuits have a tendency to produce spurious"responses or to'break into random modes of operation. For example, aline out for one-quarter wave operation will operate, spuriously inanyodd-quarter-wavelength mode. Likewise, a line out for one-half waveresonance will operate spuriously, in any integral number of halfwavelengths greater than one.

It is accordingly'an object of this invention to providea distributedconstant radio frequency circuit which is substantially free of theabove defects.

Itis" another object of this invention to construct a simple,inexpensive distributed constant radio frequency circuit offeringexceptionallywide range tunin and mode stabilization at frequenciesabove 300 megacycles.

Other objects and features of the presentinvention willbecome apparentupon a careful consideration of the following detailed description whentaken together with the accompanying drawings, in which:

Fig; l is a partial cross-sectional view taken along thelongitudinalaxis of one embodiment of the present invention;

Fig; 2 is'a cross-sectional View taken through the plane 22 of theembodiment illustrated in Fig. 1.

In accordance with the basic concepts of the present invention, adistributed constantradio frequencycircuit is constructed whichcomprises aparallel line section dimensioned to operate, fundamentally,in a greater than unity odd quarter wavelength mode. In the single;specialized embodiment herein exemplified a threequarter was/elengthmode is selected. The. line section is constructed of a pair ofdissimilar section so dimensioned, electrically, that the discontinuityformed at their juncture produces Wave reflections which combine'withthe: wave reflectionsproduced at one end of the line in an additivemanner when the line isexcited in its selectedmode'. This action assureseflicient, stable operation of the circuit.

Conversely, the phase relationship between the wave reflections fromthe: end of: the. line and 2. from the discontinuity is such, when theline is excited at any frequency other than the three quarter waveresonant frequency, that substantially all the harmonics are eithercancelled or so thoroughly attenuated as to produce negligible effect ontheoperationof the circuit.

A suitable resistance such as a coating of aquadag may be applied to theline at any convenientpoint therealong to further damp out spuriousmodes of operation.

Tuning of'the line may, as will. hereinafter be described, be attainedby using the novel tuning principles disclosed in my co-pendingapplication, supra.

Referring. nowin particular to Figs. 1 and 2 there is-disclosed thenovel teachings of my invention as applied to a modified concentric lineoscillatorwhich comprises avacuum tube component IE), and a line sectionincluding inner and outer conductors H and I2 respectively. In thisparticular embodiment th line is dimensioned to produce operation in athree quarter wave mode. The-inner conductor I l of the line istransversely slotted over a portion of its length Ila and is maderotatable fortuning reasons as will hereinafter be described. Innerconductor H is also made in two sections, one section Ila of a halfwavelength long and the other lib of a quarter wavelength long. Thesedimensionsare taken at the-highest frequency of operation. The firstsection I la is made with a relatively large diameter whereby itcooperates with the outer cylindrical conductor I2 to form a half waveresonant line of low-characteristic impedance. The second section Ilb ismade with a relatively smalldiameter whereby it may cooperate with theouter conductor I2 to form a quarter wave line section of relativelyhigh characteristic impedance. The two line sections are effectivelyinseries with one end shorted through a radio frequency bypass Whichcomprises a fiber Washer l4 and a sheet of mica IE to the end wall l3 ofthe outer cylinder I2.

In operation in the three-quarter wave mode a voltage maximum'will existat the juncture of line sections Ila and llb. Conversely a voltageminimumwill exist at the shorted end of section Hb. At-thisfrequency,line section Ila, will appear to be one-half wavelength long terminatedin the infinite impedance reflected by the quarter wave section ll b.The wave energy reflected from the discontinuity in the line, that isfrom the juncture of line sections Ila and llb will be reflected withoutphase inversion since the half wave section I la is terminated in aninfinite impedance. The energy reflected from the lower end, theshortedend of quarter wave section Hb,

Il therebetween is critical. 1%" slots with is" spacing gave a tuningrange however, will be inverted since the line section is terminated ina short circuit. Now then since the propagation path extending from thejuncture of sections Ila and MD to the shorted end of section I lb andback is equal to one-half wavelength in the desired mode of operation,and since the energy reflected from the shorted end of section llb isinverted the reflected energies from both the discontinuity and from theshorted end of the line will combine in phase. This phase relation isthat which is required for efiicient, stable operation of the circuit.

In the quarter wave resonant mode the reflections occurring from the endof the line are out of phase with those occurring from thediscontinuity. As a result of this difference in phase in the tworeflections, the oscillations in this mode are seriously attenuated.Similarly, in the five quarter wave mode and all other odd quarter wavemodes (except possibly the nine quarter wave mode), the two sets ofreflections (one set from the shorted end of the line and the other fromthe discontinuity) are out-of-phase giving little chance for sustainedor efiicient operation of the circuit except in the three quarter wavemode.

To further assist in the damping out of spurious modes of operation, Ihave found it desirable to coat a thin layer of resistance material l6such as aquadag on one of the conductors at the juncture of the two linesections. In the three quarter wave mode, a current minimum exists atthis point and little or no energy is dissipated in the resistance. Inall other spurious modes some predetermined small current exists at thispoint thus giving rise to a voltage drop across the resistance whichfurther accentuates the attenuation of these modes.

In order to permit tuning of the present circuit and in accordance withthe principles disclosed in my aforementioned co-pending application,the inner conductor H is made rotatable relative to the outer conductorI2 and is disposed off-axis from outer conductor l2. Likewise the innerconductor is transversely slotted at uniformly spaced intervals I! alongthe length I la.

This construction provides a line wherein the distributed capacitancebetween conductors I2 and II is constant with rotation of conductor II,but the distributed inductance is variable with rotation of theconductor II. This characteristic arises by virtue of the fact that thefringe capacity between the teeth E8 of the slotted portion of conductorII and the inner wall of conductor I2 is essentially equal to thedistributed capacity between the solid portion [9 of conductor II andconductor l2. Thus the distributed capacity of the line will remain thesame with rotation of conductor II.

The mutual inductance between conductors II and I2, however, varies withthe spacing between the solid portion I9 of conductor II and conductorl2. Accordingly, as conductor H is rotated the distributed capacitybetween conductors remains constant while the mutual inductance betweenconductors varies, it being a maximum in the position illustrated inFig. 2. In this condition, the resonant frequency of the line section isa maximum. Rotation of conductor ll 180 minimizes the mutual inductanceand also the resonant frequency of the line section.

In the construction of the inner conductor neither the width of theslots [8 nor the spacing In one embodiment of nearly two to one. 2"slots with spacing was also found to give a tuning range ofsubstantially two to one. Similarly, the angle of the solid portion ofconductor H is not critical and can be varied within wide latitudeswithout impairing the operation and performance of the circuit, sincethis angle only determines the value of self inductance of theconductor. The greater the angle, the smaller the self inductance andthe higher the resonant frequency.

In this application .of the invention the inner conductor II is hollow,and is provided with a hollow reentrant sleeve 23 the extension of whichforms the section ll'b. Sleeve 23 is positioned over a suitable centerpost 3 I. Spatially separating center post 3| and reentrant sleeve 23 isa fiber washer 24. Washer 24 as Well as the air dielectric between post3| and sleeve 23 operate to provide a radio frequency connection betweenthe center post and the inner conductor il. Post 3| is ofi-set from theaxis of the outer cylinder l2 sufficiently to permit a good capacitiveassociation between the inner conductor H and the inner walls of theouter cylinder 12.

A gear wheel 25 is securely attached to the lower end of the innerconductor ll for engagement with a fiber spur gear 26. Spur gear 26 ispinned or otherwise attached to a knurled knob 21 that projects from endwall 28 of the outer cylinder to provide rotation of the inner conductorII and therefore tuning of the line. Insulatingly separating the gears25 and 26 from the end Wall 28 is a strip of mica insulation [5. Themica l5 coacts with the end wall 28 and the lower end of cylinder II toprovide a capacitive short circuit between the ends of the inner andouter conductors.

The upper end of the center post 31 is longitudinally slotted at 30 toprovide a finger like contact engagement with the plate cap 31A of alight house tube [0. The lower end of the center post is anchored to endplate 28 through pin 42, which also serves as a connection for a sourceof plate supply potential.

The upper end of the concentric line is closed by a cup-shaped end cap33 held to the outer cylinder l2 by screws 60 and apertured at 34 topermit entry of the tube into the line. Attached to the inner peripheryof aperture 34 is a flexible annular contact ring 35 adapted to contactthe grid ring 36 of tube l0 thereby to provide grounded grid operationof the tube.

Surrounding the cathode ring terminal 39 of tube 10 is an annularsupport 31 held in position :by a plurality of stand-off insulators 43.Attached to the ring support 31 is a flexible annular contact ring 38adapted to provide contact with the cathode of the tube I0.

To provide regenerative feedback for the production of oscillations, acapacitative feedback strap 40 is attached to the cathode support ring31 and lead down through an aperture 4| cut in cap 33 to capacitativelyengage the anode of tube Ill. The output may be derived from theoscillator by means of an inductive loop 50, for example, disposed onthe end walls 28.

From the foregoing it becomes apparent that I have provided adistributed constant circuit which is of simple and inexpensiveconstruction and which is easily tunable by a simple rotation motion. Itis further apparent from the foregoing that numerous modifications maybe made of the present invention without exceeding the spirit thereof.For example, the principles of this invention may be applied with equaladvantages to other types of line controlled circuits such as the twowire line circuit now commonly in use. Also the invention may be used asa wave trap, or for radio frequency amplification or mixing as well asfor the production of mode stabilized oscillations. Accordingly, it mustbe understood that this invention is not to be limited except as far asdefined by the appended claims.

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

What is claimed is:

1. A radio frequency circuit comprising, a pair of substantiallycoextensive, parallel conductors dimensioned to form a resonant line ofan odd multiple, greater than unity, quarter wave lengths long, saidresonant line being shorted at one end and opened at the other and beingformed from a pair of serially connected line sections having dissimilarcharacteristic impedances, the line section adjacent the shorted end ofthe resonant line having a characteristic impedance higher than that ofthe other line section, one of said line sections being shorter than theother by an odd number of quarter wavelengths.

2. A radio frequency circuit comprising, a pair of substantiallycoextensive parallel conductors dimensioned to form a three quarterwavelength resonant line shorted at one end and open at the other, saidline being formed by the series connection of a quarter wave section ofrelatively high characteristic impedance and a half wave section ofrelatively low characteristic impedance, the free end of said quarterwave section being shorted.

3. A radio frequency circuit comprising, a pair of substantiallycoextensive, parallel conductors dimensioned to form a resonant line ofan odd multiple, greater than unity, quarter wavelengths long, saidresonant line being shorted at one end and open at the other, and formedfrom a pair of serially connected line sections having dissimilarcharacteristic impedances, the line section adjacent the shorted end ofthe resonant line having a characteristic impedance higher than that ofthe other line section, one of said pair of line sections being shorterthan the othor by an odd number of quarter Wave-lengths and a coating ofresistance material applied to said line at a point intermediate theends thereof.

4. A radio frequency circuit comprising, a pair of substantiallycoextensive, parallel conductors dimensioned to form a resonant line ofan odd multiple, greater than unity, quarter wavelengths long, saidresonant line being shorted at one end and opened at the other, andformed from a pair of serially connected line sections having dissimilarcharacteristic impedances, the line section adjacent the shorted end ofthe resonant line having a characteristic impedance higher than that ofthe other line section, one of said pair of line sections being shorterthan the other by an odd number of quarter wavelengths and a coating ofresistance material applied to said line at the juncture of said pair ofline sections.

5. A radio frequency circuit comprising, first and second transmissionline sections having different characteristic impedances connected incascade, each of said sections being at least a quarter wavelength longat a predetermined frequency, means terminating the free end of thesecond line section in an impedance other than its characteristicimpedance to form a resonant line from said sections, the characteristicimpedance of said line sections having relative values operative toproduce a reflection at their junction point which reinforces thereflection which occurs at the end of the second line section when theline is excited at a predetermined frequency.

FRANK C. ISELY.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,963,723 Sterba June 19, 19342,270,949 Hulster Jan. 2'7, 1942 FOREIGN PATENTS Number Country Date428,258 Great Britain May 5, 1935

